The invention relates to the automatic cleaning of ion sources inside mass spectrometers, especially the cleaning of ion sources in which the ions are generated by matrix-assisted laser desorption (MALDI).
Desorption ion sources, particularly ion sources for ionizing samples by matrix-assisted laser desorption (MALDI), are increasingly being used for the ionization of large molecules such as large biomolecules or synthetic polymers. At the same time, higher and higher sample throughput is being demanded, at least in some fields of application of molecular biology and medical diagnostic research. The sample support plates now usually hold 384, often even 1536 samples; the analytical method involves bombarding each sample with several hundred laser shots.
In MALDI ion sources, each bombardment of the samples, which contain large amounts of matrix substances in addition to the analyte substances, with the pulses of laser light, generates a plasma cloud, from which the ions formed are then withdrawn by switching on an accelerating field. In some cases, the plasma cloud also contains solid or liquid splash particles from the quasi-explosion of the matrix material. The plasma cloud expands further; part of the material (matrix substance and analyte substance) which is vaporized or splashed in this way is deposited on the accelerating electrodes in the process. After several hundred thousand shots, i.e. after the through passage of some ten to fifty sample support plates, each containing 384 samples, visible coatings have formed on these electrodes. These insulating coatings become electrically charged and interfere with the accelerating or focusing process for the ions. The coatings must therefore be removed.
Modern mass spectrometers are equipped with automatic feed systems for sample support plates. They can thus also work through the night or even over the weekend with thousands of samples. However, the contamination problem prevents these automatic feed systems being operated at full capacity.
The method used almost exclusively until now for removing this coating has been to clean the electrodes manually after venting and opening the ion source. The cleaning is usually carried out with solvents such as ethanol or acetone. After opening the ion source housing, it is generally possible to clean the first accelerating electrode without removing the ion source; but even then, cleaning and restoring a good vacuum takes several hours, and after the mass spectrometer has been started up again it must often be readjusted, and generally a complete recalibration of the mass scale from the flight times must be carried out. If the ion source has to be removed for cleaning, the method takes even longer and requires even more extensive adjustment.
A recent proposal (A. Holle, J. Franzen, DE 103 16 655.6) uses a specially designed cleaning plate having precisely the same shape as the sample support plate to clean the first accelerating electrode by spray-washing with solvent or brushing. It is not only the first accelerating electrode which is contaminated, however. More distant accelerating electrodes are also affected. The more distant accelerating electrodes stay uncontaminated for much longer, but when the instrument is in operation for a long time with high throughput, they too have to be cleaned.
Instead of the accelerating electrodes, this type of desorption ion source can also incorporate a set of guiding electrodes of a different type. The accelerating or guiding electrodes located in an ion source are grouped together under the term “ion source electrodes” below.
It is not only MALDI ion sources which have to be cleaned, however. In mass spectrometry, cleaning the ion sources is a process which has to be carried out for all types of internal ion sources and all types of mass spectrometers. The precise cleaning frequency is determined by the application but it almost always has to be done on a regular basis. The increased use of mass spectrometers by molecular biologists and medical professionals means that this is a process which constantly has to be carried out by the manufacturers' service technicians. A solution is therefore required which maximizes the automation of the cleaning without having to open the mass spectrometer and without requiring special technical skill.